1. Field of the Invention
This invention relates to a power conversion system, and more particularly to a power conversion system wherein a plurality of unit converters are operated connected in parallel.
2. Description of the Related Art
FIG. 7 is a layout diagram of the main circuitry of one example of a prior art power conversion system. In this FIG., 1 is an AC load, and 2 to 4 are capacitors. 5 to 8 are unit converters, each of which converts a DC power to an AC power. Their respective AC terminals are connected in common, being connected to AC load 1 so as to perform parallel operation. Capacitors 2 to 4 are employed to absorb the switching surges of converters 5 to 8. 9 to 32 are self-turn-off switching devices that constitute converters 5 to 8. Hereinbelow, the case where gate turn-off thyristors (hereinbelow simply called GTOs) are employed as the self-turn-off switching devices is described. 33 to 40 are DC reactors for smoothing the DC current. 41 to 44 are DC power sources.
FIG. 8 is a layout diagram of the control circuit that controls the main circuit of FIG. 7. In this FIG., 59 is a current instruction value generating circuit for converters 5 to 8. 60 is a phase detection circuit. 61 is a triangular wave generator. 62 is a comparator. 63 is a logic circuit that generates an output instruction of AC currents of converters 5 to 8.
FIG. 9 is a waveform diagram given in explanation of the operation of the prior art example. Hereinbelow, the operation of the prior art example will be described with reference to FIG. 7 to FIG. 9. In FIG. 8, current instruction value generating circuit 59 generates an amplitude instruction value S1 and a phase angle instruction value of the AC current. Amplitude instruction value S1 is supplied to comparator 62.
Phase angle instruction value is supplied to phase detection circuit 60 and triangular wave generator 61. Signals S2 to S5 are the output signals of triangular wave generator 61, and are triangular waves Whose period is 60.degree. of the phase angle instruction value. Triangular waves S3 to S5 lag in phase by 15.degree. in each case respectively with respect to triangular wave S2, for improving the waveform of the AC output currents of the power conversion system composed of multiple converters 5 to 8. Comparator 62 compares amplitude instruction value S1 with triangular waves S2 to S5; the range in which amplitude instruction value S1 is larger than triangular waves S2 to S5 is taken as the output instruction for the AC current. Output instruction from comparator 62 and output of phase detection circuit 60 are supplied to logic circuit 63, which Generates output instructions S6 to S13 of the AC currents of each of converters 5 to 8 by phase discrimination using the output of phase detection circuit 60.
In more detail, S6 is a U-phase output instruction of converter 5. S7 is an X-phase output instruction of converter 5. S8 is a U-phase output instruction of converter 6. S9 is an X-phase output instruction of converter 6. S10 is a U-phase output instruction of converter 7. S11 is an X-phase output instruction of converter 7. S12 is a U-phase output instruction of converter 8. S13 is an X-phase output instruction of converter 8.
The GTOs 9 to 32 of converters 5 to 8 are on/off controlled in accordance with the above output instructions S6 to S13, so that a squarewave waveform current as shown at S14 in FIG. 9 is obtained as the U-phase output current. S15 is a UV-phase line voltage of AC load 1. Control is exercised in like manner for the V-phase and W-phase, delaying the phase by 120.degree. in each case with respect to the U-phase. S16 to S19 are DG voltages of the converters 5 to 8 when operation is conducted as above. S16 is a DC voltage of converter 5, S17 is a DC voltage of converter 6, S18 is a DC voltage of converter 7, and S19 is a DC voltage of converter 8.
However, when operation is conducted with the AC terminals of converters 5 to 8 connected in parallel and their conduction phases offset by 15.degree. in each case, as described above, the mean values of the DC voltages S16 to S19 of converters 5 to 8 are not equal as shown in FIG. 9. Moreover, it is necessary to provide DC power sources 41 to 44 independently for each of converters 5 to 8, so that the DC power source circuitry is complicated.